Solder flux



Patented A r. 15, 1941 SOLDEB FLUX Mike A. Miller, Arnold, Pa., assignorto Aluminum Company of America, Pittsburgh, Pa., a corporation ofPennsylvania v No Drawing. Application November 30,1939,

Serial No. 306,388

15 Claims.

This invention relates to a solder flux, and it is especially concernedwith a flux composition that is adapted to the soft soldering ofaluminum and its alloys.

The soldering of aluminum and alloys contaming aluminum (both aluminumbase alloys 7 containing more than 50 per cent aluminum as well as thosecontaining smaller amounts of aluminum), and similar metals, is rendereddifficult by reason of a natural oxide film which is formed on thesurface of the metal, and which, therefore, interferes with a wetting ofthe underlying metal by the solder. This difficulty is even morepronounced in the case of aluminum articles that have been artificiallyoxide coated, or oxide coated and dyed. In the case of dyed coatings,care must also be exercised to minimize color changes in the region ofthe soldered joint.

In order to establish a metal-to-metal contact between the metal beingsoldered andthe solder, the oxide film must either be mechanically orchemically removed or penetrated. It is generally most convenient toemploy a chemical means for removing the oxide film, said means beingusually referred to as a flux.

One of the important properties which characterizes a good flux is thatit should only superflcially attack the metal, and yet it should loosenthe oxide film, and then dissolve or otherwise displace any loosenedoxide particles. In contrast to this,-a common type of flux containing alarge amount of a heavy metal chloride, known as the "reaction type offlux, usually attacks aluminum too severely and leaves a thick depositof heavy metal without a proper removal of the oxide particles, as wellas failing to act uniformly over the entire surface being soldered.Another highly desirable property in a flux is the ability to cause aspreading of the solder over the prepared surface. Still anotherdesirable property which is important in the soldering of aluminum isthat the flux should not leave a corrosive residue. Such residues,whether on the exterior of the soldered joint or within the soldermetal, tend to promote electrolytic corrosion between the solder and thealuminum, and may thus cause ultimate failure of the joint underunfavorable atmospheric conditions. Furthermore, in many cases it isdifficult, if not impracticable, to wash or otherwise remove the fluxresidue, and hence considerable effort has been made to provide fluxcompositions that leave non-corrosive residues or no residue at all.

In order to avoid the deficiencies of salt fluxes,

the use of certain organic compounds has been suggested. Among theorganic substances which have been proposed as fluxes for solderingaluminum, and which leave no corrosive residues, is the group ofsubstances known as the aromatic amines. These compounds are said tohave a tendency to dissolve or displace aluminum oxide, but this actionis generally much too slow for practical use. It has therefore'beenproposed to form a hydrohalide acid addition compound with the aromaticamine that will decompose on being heated and thus yield a freehydrohalide gas to etch the metal surface. Although fluxes ofthis'nature leave non-corrosive residues, the attack on the aluminum bythe acid gas may be severe, and it may even cause a perforation of verythin sheets of metal.

Fluxes containing these compounds are also apt to attack the metalnon-uniformly over a large surface, with the result that an .unsoundsoldered joint is produced. Furthermore, a high temperature is generallyrequired to produce a decomposition of these compounds, and hence theyare unsuitable for the soft or low tempera,- ture soldering of aluminum.Fluxes containing these compounds, therefore, have not been entirelysatisfactory for soldering aluminum, especially in the case of articlesthat have received an artificial oxide coating.

It is an object of my invention to provide organic soldering fluxeswhich overcome, the disadvantages of the known fluxes enumeratedhereinabove. Another object is to provide organic fluxes for solderingaluminum which con tain an amine group and yet will etch an aluminumsurface in a uniform manner, even though the surface be covered with aheavy oxide coating. Still another object is to provide a flux that isparticularly adapted to the soldering of very thin sheets of metal. Afurther object is to provide a flux which enhances the spreadability ofmoltensolder.

My invention is based upon the discovery that a, greatly improved fluxfor soft soldering is ob tained by using boron-fluorine-containingaddition compounds of certain amines. More particularly, I have foundthat the poly-amines. their derivatives, and substitution products whichcontain at least two carbon atoms combined with one or moreboron-fluorine-containing compounds, such as boron trifluoride,hydrofiuoboric acid, and heavy metal fluoborates, form highly usefulfluxes for soldering aluminum and similar metals. These additioncompounds of the poly-amines have proved to be very efiicient inpreparing the surface to be soldered, and in promoting the spread of themolten solder. Furthermore, these compounds attack the metal surfaceuniformly without danger of excessive pitting or perforation in the caseof very thin sheet material. This behavior of the poly-amine additioncompounds is in marked contrast to the severe action of the hydrohalideaddition compounds of the aromatic amines, as well as their failure toprepare the surface of an artificially oxide coated aluminum article forsoldering For the sake of convenience, the poly-amine addition compoundsmentioned above will be referred to as boro-fiuoridic-poly-amineaddition 'compounds.

The poly-amines which unite with the boronfluorine compounds to form theessential ingredient of the soldering flux belong to the group of aminesthat contain two or more amine groups. The amine groups may or may notbe connected with the same carbon atom. The poly-amines that are usefulfor flux purposes also contain two or more carbon atoms. Some of thepoly-amines which have been found to have a very satisfactory fluxingaction on aluminum incombination with the added boron-fluorinecompoundsv of a single poly-amine is an effective fluxing agent, it issometimes difiicult to obtain a pure poly-amine in commercialquantities, the commercial product actually being a mixture of two ormore amines. I have found that such mixtures'provide a base equally assatisfactory for reaction with a boron-fluorine compound as a singlepoly-amine. In other words, I have found that mixtures of boro-fluoridicaddition compounds of different poly-amines also form satisfactoryfluxes. For this reason, I consider that the poly-amines which formaddition compounds with boron-fluorine-containing compounds constitute agroup of chemically closely related substances, any member of the groupor combination of members being equivalent to the others and all beingparticularly adapted to the manufacture of a solder flux.

For some p p ses it is desirable to employ boron-fluorine additioncompounds of the polyamines along with'the amino alcohol additioncompounds in making a flux. The amino alcohol addition compoundsconstitute another group of substances which are in many respectssimilar to the poly-amine addition compounds, and are more fullydescribed in my copending application, Serial No. 306,887. Thus, inreferring to fluxes containing poly-amine addition compounds, it is :tobe understood that the flux may also contain amino alcohol additioncompounds.

Although certain hydrofluoric acid and other hydrohalide acid additioncompounds of the aromatic amines have been proposed for aluminum solderfluxes, as mentioned hereinabove, it has been found that they attack themetal nonuniformly and are active only at higher temperatures than areusually employed or are necessary in soft soldering. I have nowascertained that the use of boron in combination with fluorine in apoly-amine addition compound modifies the action of the fluorine to theextent of rendering the attack on the metal niore uniform and morespecific, and causing it to occur at lower temperatures. Furthermore, itdoes not appear that the presence of other elements or compounds in themixture along with the boron and fluorine interferes with the action ofthese two elements composition.

pounds are to be used in the flux along with upon themetal beingsoldered and upon the spreading of the molten solder.

The fluxes which I have found to be so efiicient for soldering mayconsist of a single boron-fluorine-containing compound combined with apolyamine in equimolecular proportions, such, for example, as diethylenetriamine boron trifluoride. It is also desirable at times to useaddition compounds containing two or more molecules or even a fractionalpart of a molecule of a boron-fluorine-containing substance, themolecules being of the same or different boro-fluorides. For somepurposes, the combination of hydrofluoboric acid and a heavy metalfluoborate with the amine is extraordinarily effective, and hence thisis used in preference to an addition compound containing either one ofthe boronfiuorine-containing substances. Fluxes containing fluoboratesof the heavy metals; zinc, cadmium, tin, lead, and the like, areparticularly useful in the soldering of aluminum. It is preferable toemploy between 0.5 and 25 per cent of any one of these fluoborates, butthe total should not exceed about 25 per cent. precise relationshipbetween the heavy metal fluoborate and the amine is not definitelyknown, there is some evidence of the formation of a complex compound.However, I have definitely determined that the addition of a heavy metalfluoborate to a poly-amine, particularly in conjunction withhydrofiuoboric acid, produces a highly satisfactory flux.

While it is possible to use a boro-fluoridic polyamine compound as aflux without the addition of any other substance, such as a vehicle or acarrier, I have found it to be more desirable to dissolve this compoundin an excess of the amine, especially since these compounds are readilysoluble in the amino alcohols and poly-amines. For example, a fluxcomposed of 10 per cent of the hydrofluoboric acid addition compound ofdiethylene triamine dissolved in diethylene triamine is satisfactory forsoldering aluminum. Other solvents than the aforementioned amines may beused, but where aluminum is to be soldered, especially if it hasreceived an artificial oxide coating, it is highly advantageous to use apoly-amine. In general, I prefer to use from 1 to 100 per cent of theaddition compound where it is to be the only compound of this kind thatis added to a flux carrier or vehicle If two or more addition comothersubstances, at least 0.5 per cent of each amine addition compound may beused. The foregoing percentages as well as those given hereinbelow referto percentage by weight of the entire fiux composition.

In soldering aluminum with certain solder compositions, I have found itto be more satisfactory to employ a mixture of hydrofluoboric acid and aheavy metal fluoborate in combination with a poly-aminedissolved in anexcess of diethylene triamine than to use either compound alone. In suchcases where one of the addition compounds contains hydrofiuoboric acid,I prefer to use from 5 to 50 per cent of this compound and from 0.5 to25 per cent of the other compound. Since all of the addition compoundsare, to some extent, soluble in the amine base, it has proved to be moreconvenient to add the boron-fluorine-containing substance or substancesdirectly to the amine.

For some purposes it may be desirable to mix the boro-fluoridicpoly-amine addition com- Although the is, of course, necessary to selectthe composition amounts of 0.5 to per cent. Likewise, other I organicboron trifiuoride compounds may be dissolved in a poly-amine to give 'agood flux.

The poly-amines cannot only dissolve the addition compounds mentionedhereinabove, but

both the amines and the amine solutions are soluble in water andalcohol, which makes it possible to use water or alcohol as diluents.Generally speaking, however, alcohol is preferred to water where it isnecessary to make a more fluid ilux. Because water and alcohol may beemployed in combination with the amines, it becomes possible to usewater or alcoholic soluble substances which might not be readily solublein the amines. One of the great advantages of using boro-fluoridicpoly-amine addition compounds as a flux is that these compounds do notdecompose in contact with water or moisture at room temperature to yielda free halogen or hy drohalide gas. In other words, the amine additloncompounds which I have discovered are more stable under ordinaryatmospheric conditions, and evenin contact with water, than previousorganic fluxes containing a hydrohalide, particularly with'an excess ofamine.

When the amine addition compounds de scribed herelnabove are used in theusual mannor in soit soldering of metals, I have found that they do notleave anundesirable residue. and hence they can be safely used forsoldering aluminum. Where these compounds are not completely vaporizedor decomposed by the soldering operation, the compounds themselvesficial oxide coating.

have been found to be relatively inert toward aluminum at roomtemperature, even in the presence of moisture as pointed outhereinabove.

. For this reason, it is possible to use thesev compounds in iiuxeswhere it is impractical to wash orotherwise clean the soldered joint.

.iinother important advantage exhibited by the fluxes describedhereinabove is that they do not possess an 'ofiensive odor whichcharacterizes aliphatic amines. Since this characteristic contributes tothe health and comfort of the operator, it also contributes to theutility, of the fluxes.

As mentioned above, the amine addition'compound fluxes can be handled inthe same manner as solder fluxes have heretofore been employed. They canbe made up into the form of a solid, a liquid, or a paste, dependingupon the relative proportions of liquid and solid constituents and theaddition oi various plasticizers, such as Vaseline, stearic acid, oleicacid, paraflin, etc., the amount used generally varying between 5 andper cent. The fluxes can likewise be used with the customary solderingtools and appliances. I have found that these fluxes are particularlyuseful in soldering by the so-called hot plate method where nomechanical rubbing of the solder is employed. Because of the easewithwhich my improved flux can be used, no additional operation or specialmanipulation square foot for 15 minutes.

is required in existing equipment or methods of soldering aluminum andother metals.

The amine addition fluxes which have been described may be used with' avariety of soft solders. In the case of soldering aluminum, it

that will cause a minimum electrolytic attack between the solder anclthe aluminum. From my experience, it appears that certain amine additioncompounds are more suitable for use with a particular solder compositionthan other additions compounds. In any particular application, however,the adaptability of a particular flux to a solder can be readilydetermined by a few'tests.

Although the soldering of aluminum has been emphasized hereinabo've, itis to be understood that the amine addition compounds may also be usedin fluxes designed to solder other metals, such as copper, iron, nickel,brass, certain steels,

etc. Magnetic iron base alloys containing aluminum, such as thosedescribed in U. S. Patents 1,947,274 and 1,968,569, are easily solderedby using the flux described hereinabove. The highly emcient manner inwhich my fluxes attack metal oxides make them very useful wherevermetals are being soldered which possess a superlillustrations oi theeffectiveness of the amine addition compounds are to be found in thefollowing examples. The effect of a preferred flux composition on thespreadability of molten solder on an aluminum surface may be seen in thefollowing test. For this test, pellets of tin solder having a. meltingpoint of 395 F. and weighing 0.33 gram were placed in the center oi twoaluminum sheets 1%,, inches square and inch 1 thick. The pieces or sheetand solder were then heated on a hot plate until the solder pellet justmelted. At this stage a measured amount of a coercial zinc chloride fluxwas dropped on the solder globule on one sheet, while a litre amount ofa flux composed oi l2 grams 2st.

and 8 grams Cd(BF4)2 dissolved in grams diethylene triamine was droppedon the solder globule on the other piece of sheet. in the secondinstance, the solder spread out immediately because of the cleansing andspreading action of the flux, the diameter of the area covered bythejsolder being approximately inch. In the case where zinc chloride norhad been used, no spreading oi the solder occurred.

To show the effect of penetrating an artificial oxide film on aluminum,a square of sheet was taken that had received an artificial oxidecoating by anodic treatment at room temperature in an electrolytic bathcontaining 15 per cent suliuric acid, at a current density of 12 amperesper A pellet of solder piece of sheet, as in the forheated over a flameuntil was placed on this mer test, and both were the solder becamemolten. addition compound flux referred to above was placed on thesolder, the latter spread out over the surface of the aluminum in thesame manner as in the case oithe nonartiilcially oxide coated sheetmentioned in the foregoing example, except somewhat more slowly.

Still another test showing the utility of my flux is that wherein aportion of a piece of aluminum foil, 0.00018 inch in thickness, wascoated with the flux mentioned above and heated to a temperature ofabout 450 F. for 5 minutes. At the end of this period, the sheet waswashed in water and examined for any signs of penetra- When the aminetion of the metal by the flux. No perforations were observed in the fluxcovered area. The same observation has also been made in a number ofother tests. On the other hand, when a reaction flux, of the inorganicsalt type containing a large amount of heavy metal chloride, was placedon a piece of the same foil, and heated to thesame temperature, the foilwas perforated in less than 10 seconds.

The term aluminum as employed in the ap pended claims refers to both thepure metal as commercially produced and alloys containing more than 50per cent aluminum.

I claim:

1. A solder flux composition containing a boron fluorine additioncompound of an aliphatic polyamine.

2. A solder flux composition containing a heavy metal fluoboratecompound and an aliphatic poly-amine.

3. A solder flux composition containing a heavy metal fluoboratecompound and a hydrofluoboric acid addition compound of an aliphaticpolyamine.

4. A solder flux composition containing a boron fluorine additioncompound of an aliphatic polyamine and a heavy metal fluoride.

5. A solder flux composition containing a boron fluorine additioncompound of an aliphatic polyamine and at least one substance selectedfrom the group of plasticizers composed of Vaseline, stearic acid, oleicacid, and paraffin.

6. A solder flux composition containing a boron fluorine additioncompound of an aliphatic polyamine and uncombined poly-amine.

7. A solder flux composition containing from 0.5 to 25 per cent cadmiumfluoborate and 5 to 50 per cent hydrofluoboric acid addition compound ofan aliphatic poly-amine and uncombined poly-amine.

8. A solder flux composition containing from 0.5 to per cent cadmiumfluoborate and 5 to per cent hydrofluoboric acid addition compound of analiphatic poly-amine, uncombined poly-amine, and a plasticizingsubstance.

9. A'solder flux composition containing a boron fluorine additioncompound of at least one of the poly-amines oi the group consisting ofethylene diamine, diethylene triamine, triethylene ,tetramine andltetraethylene pentamine.

10. A solder flux composition consisting of 0.5 to 25 per cent cadmiumfluoborate, 5 to 50 per cent hydrofluoboric acid addition compound ofdiethylene triamine and uncombined diethylene triamine.

1. A solder flux composition containing a heavy metal fluoborate and analiphatic poly-v amine', said heavy metal fluoborate being selected fromthe group composed of the fluoborates oi zinc, cadmium, tin, and lead.

12. A solder flux composition containing a boron fluorine additioncompound of an aliphatic poly-amine and a fluoride of at least one ofthe metals of the group consisting of zinc, cadmium, lead, and tin.

13. A solder flux composition containing 0.5 per cent each of at leasttwo boron fluorine addition compounds of diiferent aliphaticpoly-amines.

14. A method of soldering an oxide coated metal comprising applying asolder flux to the area to be soldered, said flux containing a boronfluorine addition compound of an aliphatic poly-amine, and bringingmolten solder into contact with said flux.

15. A method of soldering artificially oxide coated aluminum comprisingapplying a flux to the area to be soldered, said flux containing ahydrofiuoboric acid addition compound and a heavy metal fluoborateaddition compound of an aliphatic poly-amine, and bringing molten solderinto contact with said flux.

- MIKE A. MILLER.

